Earthrace Conservation founder Pete Bethune said a week of "covert surveillance" by plane, boat and kayak this year found illegal fishing in the Banks Peninsula Marine Mammal Sanctuary.

The information had been passed on to the Ministry for Primary Industries.

The ministry said it was investigating activity in the area and would carry out a review of the sanctuary this year.

Bethune and eight volunteers carried out the operation because they were concerned Hector's dolphins were being caught and killed as bycatch. Bethune said commercial vessels were breaching sanctuary rules when gill-netting and trawling, and both methods killed the endangered dolphins.

"We are just relying on fishermen's honesty to preserve this [sanctuary] and from what we saw last week, we can't trust them. All it takes is a few scumbags. At the moment it is not a sanctuary; it is a killing field."

He said the Hector's dolphin population was rapidly heading towards extinction through being caught in fishing nets and a lack of effective response to the threat by the Government. There are about 7000 Hector's dolphins in New Zealand's waters, with about 1000 of the nationally endangered species off Banks Peninsula.

Bethune wants the ministry to extend the exclusion area for gill-netting and trawlers to up to 20 nautical miles from the coastline and increase the number of fisheries inspectors in all of New Zealand's marine mammal sanctuaries.

Ministry deputy director-general Scott Gallacher said anyone with evidence of illegal fishing activity should contact the ministry so it could conduct a proper investigation.

"[The ministry] takes any breaches of fishing law very seriously and is actively investigating this allegation."

Fisheries officers boarded a "vessel of interest" and seized items last week, but he would not comment further as the investigation was continuing.